Scraper rings are intended to seal the piston of a rotary piston engine in relation to the end wall of the housing so as to prevent coolant or lubricant from passing radially outward into the working chamber. To perform this function known scraper rings act essentially to scrape off the oil adhering to the end wall of the housing and bring it back radially inwardly. For this purpose, the scraper rings on their face adjacent to the end wall of the housing exhibit at least one concentric annular scraping edge, generally sharp edged radially inwardly, while a flat bevel is provided radially outwardly. Owing to this conformation, in centrifugal movements of the scraper ring in a radially outward direction the oil clinging to the end wall of the housing is run over through formation of a hydro-dynamic supporting wedge, and in centripetal movements it is scraped off and brought back radially inwardly.
In known manner, the scraper ring revolving with the piston traverses equal areas of the end wall of the housing inwardly and outwardly alternately in the course of a full revolution. In the phase of inward-directed motion the scraper ring will indeed scrape the traversed area clear, but some of the oil brought inwardly may cling along the scraping edge assisted by centrifugal forces. Now, although a certain quantity of oil may be circulating radially within the scraper ring in the ensuing phase of outward-directed motion the scraper ring must lift off, running over and bring back more oil clinging to the end wall of the housing despite the oil being entrained centrifugally outwardly. It follows from this cycle that if the design of the scraping edge is not efficient, dependable functioning of a scraper ring will cease to be assured. A durable and dependable seal will therefore depend very substantially on the conformation of the scraping edge; it may also be adversely affected by manifestations of wear and dimensional errors, as well as by warping of the end wall of the housing.